The invention is based on a planar sensor element for determining the oxygen content of gas mixtures. The sensor element comprises an integrated heat conductor embedded in electric insulation resting on a solid electrolyte substrate. Sensor elements of this type are used in electrochemical measuring sensors and probes, such as those used to determine the oxygen content of gases and the .lambda. value (total oxygen over oxygen required for complete fuel combustion) of gas mixtures, particularly in internal combustion engines.
Planar sensor elements have proven advantageous in practice because of a simple and cost-effective method of production involving the use of wafer- or film-shaped solid electrolytes, that is, ion-conductive materials such as stabilized zirconium dioxide. Planar polarographic sensor elements and probes that operate according to the diffusion resistance principle have achieved particular significance in practice. Sensor elements and probes of this type are known, for example, from German Published, Non-Examined Patent Applications 3,543,759 corresponding to U.S. Pat. No. 4,839,018 and U.S. Pat. No. 5,098,549, and 3,728,618, as well as from European Patent Applications 0,142,992, 0,142,993, and 0,194,082, and also from European Patent Application 0,148,622 corresponding to U.S. Pat. No. 4,610,741.
In polarographic probes of this type, which operate according to the diffusion resistance principle, the diffusion current at a constant voltage present at the two electrodes of the probe, or the limiting diffusion current can be measured. In an exhaust gas produced during combustion processes, the limiting diffusion current is a function of the oxygen concentration for as long as the diffusion of the gas toward the pumping electrode determines the rate of the occurring reaction. It is known to construct such polarographic probes, which operate according to the polarographic measuring principle, in such a way that both the anode and cathode are subjected to the gas to be measured, with the cathode exhibiting a diffusion barrier.
In their preferred embodiments, the known electrochemical measuring sensors and sensor elements have heater units or heat conductors. This also applies, for example, for the planar polarographic probe known from German Published, Non-Examined Patent Application 3,811,713, corresponding to U.S. Pat. No. 5,169,512, which has a pump cell (A) and a diffusion unit (B) having a diffusion resistance upstream of the pumping electrode of the pump cell (A), in which the diffusion resistance of the diffusion unit (B) is formed by a porous, sintered body inserted into the unsintered probe. The layouts of polarographic probes having heater units are shown, for example, in FIGS. 1 and 13 of German Application 3,811,713.
If a planar sensor element comprising a solid substrate has an integrated heat conductor, this conductor is typically embedded in a conventional manner in an insulating material, such as Al.sub.2 O.sub.3, and the heat conductor and insulating material are in turn embedded in the ion-conductive solid electrolyte material.
The disadvantage of such embedding is the danger of electrical coupling of the heater with the measuring cell(s) integrated into the sensor element. Causes of the above can be:
(1) insufficient thickness of insulation layers between the solid electrolyte and heater; PA1 (2) defective insulating layers due to holes (pinholes), breaches, defective spots; PA1 (3) a limited insulation capacity of the insulating material, and PA1 (4) a capacitative influence of the heat conductor on the measuring cell.